1. Field of the Invention
The present invention relates to ablator compositions and more particularly to an ablator composition, which utilizes an intumescent coating.
2. Description of the Related Art
Launch vehicle configurations often employ solid rocket boosters (SRB""s) to augment the thrust of the main engine. Because of the proximity of the SRB plumes to the base region of the main engine, the convective and radiative heating augmentations to the base region of the main engine due to these SRBs are substantial. A layer of low temperature ablative (LTA) is needed to protect the structure in this region. The thermal, mechanical and chemical performances of the ablatives in the hostile environment produced by the rocket exhaust are of importance in the design of the thermal protection system. The requirements imposed on the insulation are as follows:
1. Ability to withstand the aerodynamic loads and aeroheating encountered during flight;
2. Protection and maintenance of the substructure below a critical temperature;
3. Light weight, low cost and ease to manufacture and to install;
4. Ability to withstand thermal shock due to launch plume heating;
5. Ability to withstand the mechanical and acoustic vibration environment; and
6. Chemical and mechanical compatibility with adhesive and substructure throughout the entire flight.
When an external heat flux is applied to the base of the main engine, the LTA material protecting this region may decompose in-depth and recede at its surface. The modes of surface recession may include combinations of phase change processes such as melting, sublimation, as well as, exothermic or endothermic chemical reactions such as oxidation and combustion. Similarly, in-depth decomposition, such as pyrolysis, may involve outgassing, phase change and chemical reactions. The ablation performance of these LTA are often characterized by q* or
37 heat of ablationxe2x80x9d defined as
q*=qdot/mdot,
where qdot is the net heat flux=qhwxe2x88x92qrad;
mdot is the rate of mass loss;
qhw=convective hot wall flux; and
qrad=net radiative heat flux.
The primary mechanisms for the LTA to counter the applied heat flux are high heat of ablation and low thermal conduction. Thus, the ideal properties of LTA include low density and thermal conductivity, ease of manufacturing and installation, and the ability to withstand flight conditions.
Another desirable property of an ideal LTA is to form strong char during the ablation process. If the strength of the char adhering to the surface is sufficient to keep it from being swept away by aerodynamic shear forces and acoustic vibrations, the performance of the insulation can be improved because of:
a) Increased thermal protection since less material is removed;
b) Increased thermal protection since the char in general is porous, lightweight and has low thermal conductivity; and
c) Increased radiant heat loss from surface since the char in general has higher emissivity and can withstand high temperature. The higher surface temperature also reduces convective heat gain.
Cork, with over 200 million cells per cubic inch, is often chosen as the LTA thermal protection system (TPS) because of the structure and mechanics of these cells. It is used as insulation material for launch vehicles because of its low density yet resilient mechanical properties; minimal cost; its ability to absorb vibration and withstand acoustic noise; and, its chemically stability. This natural product is cleaned, ground, mixed with various resins such as phenolic and formed into complex shapes. Common cork based TPS materials include cork epoxy, cork phenolic and cork silicone.
The combustion of cork and phenolic resin to form weakened char is the single most important failure mode of the cork phenolic heatshield materials. When the material is exposed to high heat flux and oxygen from ambient atmosphere, the cork-based ablatives quickly char and begin burning. Once ignited, the ablatives will continue to burn even after the external heat source is turned off. As the cork phenolic TPS ablates, the surface of the TPS will form char with cracks, the size of which increases with time. Eventually the remaining material will break and erode away due to the mechanical load or aerodynamic shear.
A typical launch vehicle may sit on the launch pad for days prior to flight, and often the TPS can absorb a significant amount of moisture if left unprotected. Existing families of launch vehicles often employ a coating of paint to seal the TPS. The launch vehicles may also have an additional layer of electrically conductive paint to ground electrical charges in the atmosphere.
The layer of LTA needed to protect the structure from excessive convective and radiative heating can add substantial weight, cost, technical risk and performance penalties to the launch vehicle""s manufacturer and integration team.
It is therefore a principal object of the present invention to provide an improved ablative composition, which can be applied to a substrate to protect the substrate from external heat flux.
It is another object to coat an ablative material such that during exposure to heat, the coating will swell to provide a thermal barrier, inhibit ambient air from contacting the ablative material, and provide a back fill into interstices within the ablative material and char to enhance their strength.
Another object is to provide an ablative composition with a moisture barrier.
Still another object is to provide an ablative composition with a layer of electrically conductive coating to ground electrical charges in the atmosphere.
Yet another object is to provide an ablative composition with a coating to reflect incoming radiant heat flux.
These and other objects are achieved by the present invention, which in its broadest aspects comprises a thermally insulative ablative material and an intumescent coating covering the thermally insulative ablative material. During the application of heat the intumescent coating is transformed into a swollen char material, which acts as a thermal barrier to eliminate or minimize incoming heat flux. It also acts as a mass transfer barrier, inhibiting oxygen from reaching the thermally insulative ablative material. During the intumescence process, the swollen material will also back fill into interstices within the ablative material and char to enhance their strength. The intumescent coating also acts as a moisture barrier to protect the thermally insulative ablative material from ambient elements such as moisture. The intumescent coating also acts as the electrically conductive paint to ground electrical charges in the atmosphere. The intumescent coating preferably contains particulate to reflect incoming radiant heat flux.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.